This invention relates to a material for use in electronic applications where low dielectric constant is important, and a process for making same.
More particularly, this invention relates to a material useful in a printed circuit board (PCB) structure, which may include more than one conductive layer and may incorporate electrical interconnections called blind vias, or through holes, between two or more conductive layers. This invention is also well suited for use as a substrate material for surface mounted integrated circuits.
Most particularly this invention relates to an improved material and printed circuit board made thereof comprising a coupled inorganic hollow microsphere filled, reinforced polymer resin composite material which exhibits high thermal stability, flame retardance and uniform low dielectric constant.
The necessity of developing ever-increasing high speed computers has led to the exploration of new materials which would extend the electrical and thermal performance limits of the presently available technology. For high speed applications it is necessary to have extremely dense conductor circuitry pattering on low dielectric constant insulating material. Prepreg laminates for conventional circuit boards are traditionally made up of a base reinforcing glass fabric impregnated with a resin. Epoxy/glass laminates, used in current products, typically contain about 40% by weight fiber glass and 60% by weight epoxy resin, and typically have a dielectric constant, (Er), of approximately 4.2. Such a relatively high Er causes electrical pulses (signals) present in the adjacent signal circuit lines to propagate less rapidly, resulting in excessive signal delay time. For new computer systems to become faster, system cycle times must become shorter. With the next generation of computers, the delay time contributed by signal travel within the PCB's will become very significant, hence the need for lower Er laminate materials. Future products are expected to require overall Er's of 2.8 or below. Such low Er's are impossible to obtain without new materials since the Er's of conventional FR4 epoxy and common fiber glass are typically on the order of 4 and 6 respectively. The effective Er of such composite materials can usually be approximated by a simple weighted average of the Er of each individual component and its volume fraction contained in the composite.
Pure fluoropolymers such as polytetrafluoroethylene (PTFE), have Er's of approximately 2.1. However, using such a material alone in construction of a circuit board laminate is impractical, due to its generally poor mechanical properties and chemical inertness. One alternative is to use fluoropolymer as one of the components of a composite laminate material, such as the fiber in the reinforcing cloth. An example of this is the treated PTFE fabric prepreg produced by W. L. Gore and Associates, of Newark, Del. When this type of fabric is used to replace fiber glass in conventional epoxy/glass laminates, the Er drops to 2.8. However use of this fabric presents certain disadvantages. Because of the comparatively low modulus of pure PTFE, thin laminates made with these materials are not very rigid, and require special handling care. Also when laminates incorporating PTFE fabric are drilled, uncut PTFE fibers tend to protrude into the drilled holes and are difficult to remove. In order to obtain good plating adhesion, exposed PTFE surfaces must be treated using either an expensive, highly flammable chemical in a nitrogen atmosphere or by plasma processing, which must penetrate high aspect ratio through holes in order to obtain good plating adhesion. Certainly one of the biggest disadvantages of PTFE fabric laminate is cost, not only the higher cost due to additional processing requirements and equipment modification, but also the considerable cost of purchasing the prepreg material itself.
Signal transmission delay time is to the square root of the dielectric constant of the dielectric material used, as expressed by ##EQU1## where T.sub.D is the transmission delay time, Er is the dielectric constant of the material, C is the velocity of light (3.times.10E8 m/sec) and D is the length of the signal path. The equation indicates that the lower the dielectric constant, the faster the signal propagation.
Ideally, the value of the Er should approach as a limit 1.0, the value in a vacuum. Lower also reduces crosstalk between adjacent circuit lines.
Another property which is in predicting the performance of a laminated dielectric material is the coefficient of thermal expansion (CTE). It is desirable to closely match the coefficients thermal expansion in the X and the Y directions of t dielectric material to that of the adjacent , which is 17 ppm, contained in the PCB in order prevent cracking of soldered joints linking the PCB a surface mounted device, or to avoid separation of copper from the dielectric, or to prevent PCB warpage. The X and Y direction CTE's are normally by the glass fibers within the matrix. However these fibers do not control Z direction CTE. Z direction CTE must also be controlled in order to prevent cracking of copper plated through holes during heat cycling. Heat is generated in preparing or reworking solder connections, and in other manufacturing processes, and during current flow when the finished board is in operation.
One way to modify the CTE is by the use of fillers. Fillers may be linked to the matrix polymer to which they are added by the use of a coupling agent, often a silane. The coupling agent improves the bonding between the filler and the polymer, minimizing the total interfacial area, which also improves both electrical and mechanical performance.
The CTE of a prepreg dielectric material changes markedly when an inflection point called the glass transition temperature (Tg) is reached. Since the expansion rate of the dielectric material increases considerably when the Tg is reached, it is desirable for a dielectric material to have a high Tg in order to minimize stresses. Epoxy novolac based dielectric materials, for example, are considered to have a relatively high Tg, generally 150 degrees C. or greater. Other characteristics associated with high Tg often include low moisture absorption and chemical inertness. A discussion of Tg is found on pp. 559-560 of Microelectronics Packaging Handbook, Tummala et al, Ed., published by Van Nostrand Reinhold, New York, 1989.
In order for an electronic device to be marketed commercially, it is desirable for the device to equal or exceed certain flammability standards specified by Underwriters' Laboratory (UL). Antimony and halogen compounds have been added to resins in order to impart flame retardance, such as the 19%-23% Br by weight brominated polyglycidyl ether of bisphenol A in epoxy resin, which is described in U.S. Pat. No. 3,523,037,issued Aug. 4, 1970 to Chellis et al.